Methods suitable for mass production of a glass preform for use in the fabrication of an optical fiber includes a vapor phase axial deposition method (hereinafter referred to as a "VAD" method and an outside vapor phase deposition method (hereinafter referred to as a "OVPD" method). These deposition methods comprise flame hydrolyzing a glass raw material such as SiCl.sub.4 optionally containing an additive (eg. GeO.sub.2) in an oxyhydrogen flame to form glass fine particles of pure quartz (SiO.sub.2) or quartz containing the additive with an average particle size of about 0.1 micrometer, depositing the glass fine particles on a seed member to produce a porous soot preform and sintering the soot preform at a high temperature to obtain a transparent glass preform. According to the VAD method, the glass fine particles are deposited on the rotating seed member in parallel with the rotating axis of the member to continuously form the solid cylindrical soot preform (cf. U.S. Pat. No. 4,135,901). According to the OVPD method, the glass fine particles are deposited on a rotating rod member made of alumina or quartz glass from a direction perpendicular to the rotating axis of the member to form multiple thin layers of the glass fine particles (cf. U.S. Pat. Nos. 3,711,262, 3,737,292 and 3,737,293). The produced porous soot preform is then heated and sintered in an atmosphere of an inert gas such as helium at a high temperature to make the soot preform transparent to obtain the glass preform.
A practically used optical fiber is required to have low attenuation of light transmission, and it is essential for the optical fiber to have total attenuation of light transmission not larger than 1 dB/km, particularly, at wavelength of 1.30 micrometer which is used for long-distance optical telecommunication. Therefore, it is required to decrease an amount of residual water (i.e. hydroxyl groups) in the optical fiber which absorbs light with a wavelength of 1.38 micrometer and influences light transmission at a wavelength of 1.30 micrometer. FIG. 1 shows a relationship between the amount of residual water in the optical fiber and attenuation of light transmission at a wavelength of 1.30 micrometer. As is clear from this relationship, the amount of residual water should be less than 0.3 ppm to decrease attenuation of light transmission to less than 0.3 dB/km.
Since theoretical limit of attenuation of light transmission of glass material at a wavelength of 1.30 micrometer is 0.3 to 0.4 dB/km, total attenuation of light transmission at this wavelength amounts to 0.6 to 0.7 dB/km.
To reduce total attenuation of light transmission to less than 1 dB/km, it is, therefore, necessary to reduce attenuation of light transmission attributed to other factors, particularly, absorption by impurities such as transition metals (eg. copper and iron) as low as possible.
Table 1 shows an amount of impurity element which causes 20 dB/km of attenuation of light transmission at a wavelength of 0.8 micrometer.
TABLE 1 ______________________________________ Element Amount (ppb) ______________________________________ V 19 Cr 33 Mn 833 Fe 425 Co 816 Ni 712 Cu 9 ______________________________________
As seen from Table 1, it is important to reduce the amount of the impurities less than 1 ppb in order to decrease attenuation of light transmission of the optical fiber. Attenuation of light transmission due to copper at a wavelength of 1.30 micrometer is about one fifth of that at a wavelength of 0.8 micrometer.
In addition, attenuation of light transmission is also caused by bubbles present in the optical fiber. Such bubbles are formed mainly of gaseous chlorine which is used as a dehydrating agent of a soot preform or gaseous GeO.sub.2 which is added as an additive for adjusting refractive index of glass.
Recently, a large-sized glass preform for the optical fiber is produced. for example, the VAD method produces such a large glass preform that 200 km of the optical fiber is fabricated therefrom. The large-sized glass preform is required to have more homogeneous and stable composition along its length than a smaller glass preform. Particularly, GeO.sub.2 volatilizes during dehydration and sintering of the soot preform according to the following equation (I) or (II):
At a temperature higher than 800.degree. C.: EQU GeO.sub.2 .fwdarw.GeO+O.sub.2 (I)
At a temperature higher than 900.degree. C.: EQU GeO.sub.2 +2Cl.sub.2 (g).fwdarw.GeCl.sub.4 (g)+O.sub.2 (II)
where (g) stands for a gas state. Therefore, a volatilized amount of GeO.sub.2 varies with slight change of dehydration and/or sintering conditions (for example, temperature) and of the flow rate of chlorine, resulting in change of distribution of refractive index of the glass preform along its length.
For instance, according to a method disclosed in U.S. Pat. No. 3,993,454, a soot preform comprising a core made of SiO.sub.2 glass containing GeO.sub.2 is sintered by gradually introducing it from its one end in a furnace with an atmosphere of helium containing chlorine (namely, a gradient sintering method). The produced transparent glass preform has refractive index difference of 0.3% between one end and the other, which corresponds to about 5% by weight difference of the amount of GeO.sub.2.
There is known another method for sintering the soot preform which comprises introducing the whole soot preform in an atmosphere containing chlorine and gradually raising temperature to a sintering temperature at which the soot preform is converted to the transparent glass preform (cf. U.S. Pat. No. 4,338,111). Although this method is suitable for stabilization of the refractive index distribution along length of the preform, it has a disadvantage that the produced glass preform tends to contain bubbles in comparison with that produced by the gradient sintering method.